To get the best look and fit for your cushions, covers should always be slightly smaller than the cushion inner. This helps maintain a good plump shape, especially with duck feather inners which tend to settle into corners.
For a cushion inner 45cm x 45cm, the actual size of the cushion cover should be around 42.5cm x 42.5cm.
One metre of standard width (137cm) furnishing fabric is enough for three plain cushions with either zip or standard button fastening up to 45cm x 45cm size. Other styles of cushion will need more fabric.